The field of the invention relates to control of direct injection engines. In particular, the field relates to vapor recovery control for direct injection spark ignition engines.
In direct injection engines, the engine control system operates the engine in both a stratified mode and a homogeneous mode. In the stratified mode, the combustion chambers contain stratified layers of different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. In the homogenous mode, a mixture with relatively constant air/fuel properties is present throughout the cylinder volume.
Fuel vapor recovery systems are employed on motor vehicles and need to be combined with direct injection engines to reduce atmospheric emissions of hydrocarbons by storing the hydrocarbons in a canister. The canister, which is coupled to the fuel tank, uses activated charcoal for absorbing the hydrocarbons. The canister is periodically purged by passing ambient air, which desorbs the hydrocarbons, through the charcoal. The resulting air and hydrocarbon mixture subsidizes the normal mixture of air, from the intake system, and fuel, from the fuel delivery system, inducted into the engine via the engine port. The canister is then able to again store hydrocarbons allowing the process to repeat. The conventional fuel vapor recovery process must also be executed at regular intervals to assure that the canister does not become saturated.
To force ambient air through the canister, manifold vacuum is commonly used. However, manifold vacuum is a function of engine operating conditions. At certain conditions, especially when operating in the stratified mode, the manifold vacuum may not be enough to force air through the canister. Thus, many vapor recovery systems utilize a pump, or compressor, in the system to allow purging of the canister at all manifold vacuum levels. Such a system is disclosed in U.S. Pat. No. 5,054,454.
Also, conventional port injected purging systems can only form a homogeneous mixture because, when the engine inducts a mixture of air and fuel from the vapor recovery system, there is no way to form a stratified mixture. In other words, the stratified mixture depends on having an isolated region of a desired air/fuel ratio with the rest of the region progressively becoming leaner. Thus, purging is restricted to when the engine is operating in a homogeneous mode. One such system is disclosed in U.S. Pat. No. 5,245,975.
The inventors herein have recognized numerous disadvantages when applying known vapor recovery systems with direct injection engines. For example, when vapor recovery systems are utilized with direct injection engines, the requirement for purging at regular intervals is not fulfilled by simply using a pump, or compressor. Thus, operation in a stratified mode, which is advantageous to fuel economy, is limited by the necessity to purge the canister, resulting in less than optimal fuel economy. Another problem is if fuel vapors are purged during stratified operation, there will be unburned hydrocarbons in the exhaust.